1. Field of the Invention
The present invention relates to a method for removing noise from an image, more specifically a medical image, represented by a digital signal representation.
2. Description of the Related Art
Commonly images such as medical radiographic images represented by a digital signal are subjected to image processing during or prior to displaying or hard copy recording.
One of the steps in the image enhancement process is image restoration.
Digital images may be degraded by noise.
Due to the strong focus on dose limitation in radiographic imaging, more and more radiographic images are taken at a lower dose resulting in higher noise content.
Therefore image denoising is a major concern in the process of visualization enhancement of radiographic images.
Over the years multiple image denoising techniques have been published. They are formulated both in the spatial domain and in the frequency domain.
Despite of the strong progress already made, research institutes continue the search for more efficient denoising methods.
State-of-the-art denoising algorithms often make assumptions about the noise model and are therefore not generally applicable.
The challenge is to suppress the noise without creating artefacts or removal of fine image structures.
A large category of promising image denoising techniques are multiscale based methods like Multiscale adaptive thresholding, Bayesian wavelet shrinkage, etc.
European patent EP 1933272 describes a new technique for multiscale contrast enhancement based on translation difference images.
In a conventional multiscale image processing method an image, represented by an array of pixel values, is processed by applying the following steps. First the original image is decomposed into a sequence of detail images at multiple scales and occasionally a residual image. Next, the pixel values of the detail images are modified by applying to these pixel values at least one conversion. Finally, a processed image is computed by applying a reconstruction algorithm to the residual image and the modified detail images.
The new technique described in the above mentioned patent EP 1933272 provides a method wherein a reversible multi-scale detail representation is computed as a weighted sum of translation difference images.
The weighing factors and the translation offsets of the translation difference images are deducted from the multi-scale decomposition in such a way that the resulting weighted sum of the translation difference images is identical to or an approximation of the pixel values in the detail images.